Despite the development and availability of stronger and more rigid construction materials, wood is still widely used in a variety of utilitarian and decorative applications. Wood is plentiful and is a renewable natural resource with consumer appeal due to aesthetic, economic and environmental factors.
The properties of wood may be modified in the production of wood products, for example: fingerjointed and laminated timber; plywood; laminated veneer lumber (LVL); particleboard; and other manufactured wood products. Adhesives are used in the production of all of these. Often these wood products have advantageous properties in comparison to wood because the production process permits the removal of defects. In addition, the products may be produced in larger dimensions than can be cut from a tree.
Some commonly used adhesives for wood products are derived from phenol-resorcinol-formaldehyde (PRF) resins. PRF-based adhesives have excellent durability and cure without the application of external heat. PRF resins are generally used in a two-part adhesive system with one of the parts usually being formaldehyde or paraformaldehyde (a more convenient, solid form of formaldehyde). Addition of formaldehyde to the PRF resin causes the resin molecules to crosslink and thus form the adhesive bond or “cure”. Therefore, the formaldehyde component is often referred to as the “hardener” in such an adhesive system.
Both phenol and resorcinol are obtained from petrochemical sources. Replacement of some or all of these components in an adhesive system by materials sourced from a renewable, natural resource would have economic and environmental advantages.
Tannins have been used commercially for many years as a substitute for phenol-formaldehyde resins in reconstituted wood products. The tannin has predominantly been derived from quebracho and Acacia species [1], whereas pine bark tannin (PBT) has not been widely used. The use of extracts from pine bark in adhesives offers the potential to utilise a material which is currently used for horticultural applications, dumped or burnt.
A number of researchers have recognised that, in contrast to quebracho and Acacia tannin, the chemistry of the reaction of PBT with formaldehyde is more like that of resorcinol than phenol. Accordingly, PBT is expected to be a suitable substitute for some of the resorcinol in PRF resins.
To date, however, resins containing PBT have been of limited adhesive utility [2]. This is due to a number of factors including: extract variability; high viscosity; and poor adhesive strength due to low cross-linking density. In addition, the high reactivity of PBT with formaldehyde hardeners means that the prepared adhesive has a short potlife that is undesirable in industry.
Methods which have been investigated to overcome the difficulties identified above include modification of the adhesive formulation or new extraction techniques. For example, sulfite extraction of bark from Pinus radiata [3], the incorporation of ultrafiltration in the processing of the extracts [4], and the modification of tannin reactivity using acids to alter the pH [5, 6].
Pizzi and Gonzalez have noted that a completely new approach is required to overcome the problem of hardening and setting a tannin-resorcinol-formaldehyde resin if, indeed, “ . . . such a problem can be completely overcome at all” [7].
To date, the most commonly used approach to the problem of limited potlife has been the use of “honeymoon” systems [8-11]. In such systems the resin is mixed and applied to one face of the joint to be bonded and the other part of the adhesive system, the hardener, is mixed and applied to the second face. When the joint is pressed together, the two components combine and the curing reaction is initiated. This is in contrast to a standard two-part adhesive system in which the resin and the hardener are mixed together prior to application.
A disadvantage of a honeymoon-type adhesive system is that it is more difficult to apply to commercial operations than a standard two-part adhesive system. For example, the preparation of the two components prior to application may require additional formulation or mixing steps.
Known honeymoon systems that utilise PBT require the tannin component to be at a highly alkaline pH [12]. This requires the hazardous and therefore undesirable use of caustic compounds in the gluing facility. In addition, once the highly alkaline tannin component used in such systems has been mixed, it remains usable for two to four hours at most. Batch mixing of the tannin component may therefore lead to wastage.
During the mid 1980s a group led by Kreibich in the U.S. developed a honeymoon system which consisted of a mixture of PBT and sodium hydroxide solution in one part and a mixture of PRF resin and paraformaldehyde hardener in the other [6, 8, 13]. The group reported that there was a viscosity increase upon addition of sodium hydroxide to the PBT and the solution remained workable for only three to four hours [8].
U.S. Pat. Nos. 5,858,553 5,858,553 [14] and 5,912,317 5,912,317 [15] disclose hardeners for use with resorcinol resins, some of which include tannins, comprising a monocyclic or bicyclic oxazolidine together with a base, a sorbent and water.
The base in the compositions of U.S. Pat. Nos. 5,858,553 and 5,912,317 is included to “ . . . retard the reaction of oxazolidine with resorcinol resin”. Thus, the use of a base increases the time before which the mixed adhesive gels and becomes unworkable. Inorganic bases such as alkali metal or alkaline earth hydroxides are preferred and may be mixed with the hardener or, preferably, premixed with the resorcinol resin.
A drawback associated with the addition of base to PBT-containing resin is that the base promotes self-polymerisation in the resin, leading to insoluble gel and thus reducing the shelf life. In addition, the chelation of some cations by tannin can lead to insoluble precipitates. The alternative approach, the addition of base to the resin immediately before addition of the hardener, requires an additional step in the formulation of the adhesive and exposes the user to the risks of handling caustic compounds.
Addition of a base to the hardener composition, in order to retard the hydrolysis reaction of the oxazolidine, is also undesirable. Basic conditions may favour the Cannizzaro reaction, an irreversible reaction in which formaldehyde disproportionates to methanol and formic acid.
Thus, neither the use of conventional hardener compositions nor of honeymoon systems readily provides an adhesive based on PBT that meets industry standards and can be used in standard gluing techniques and with standard equipment.
It is therefore an object of the present invention to provide a PBT-containing resin and hardener composition that go some way towards overcoming the above disadvantages or at least provides the public with a useful choice.
Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.